1. Field of the Invention
This invention relates to well logging during drilling and in particular it concerns a novel method and apparatus for continuously analyzing the gases returned in the drilling mud from a well as it is being drilled.
2. Description of the Prior Art
In the drilling of an oil or gas well, a drill bit is mounted on the end of an elongated rotating drill string which turns the bit and causes it to cut away the underlying earth and rock formations. During this operation a drilling mud is continuously pumped down through the drill string and into the region around the drill bit and then back up to the surface. This drilling mud is typically made up of clays, chemical additives and an oil or water base and performs two important functions. First, it serves to cool and lubricate the drill bit and to carry drill cuttings back up out from the well. Secondly, the weight of the mud, which can be adjusted by controlling the amount of special weighting additives, serves to maintain a hydrostatic pressure to prevent pressurized fluids in the earth from blowing out through the drilled well.
During the drilling of a well, various measurements are taken from the drilling mud that is returned to the surface; and these measurements, which may optionally be taken together with other measurements made near the drill bit by MWD (measurements-while-drilling) tools, provide a log of the drilling operation which permits one to analyze the earth formations through which the drill bit is penetrating. This log is important because it enables the drilling operator to ascertain the presence of oil or gas in the formation being drilled.
In the past, continuous mud logging measurements have been made during drilling. These continuous measurements included the temperature, electrical conductivity, pH, sulfide ion content and oxidation-reduction potential of the drilling mud returned from the well. In addition, continuous measurements have been made on the returning mud to ascertain its total hydrocarbon content and to ascertain the presence of certain specific gases such as carbon dioxide and hydrogen sulfide in the mud.
Past well mud logging techniques have also made use of gas chromatography to ascertain the presence of different hydrocarbon species in the mud being returned. As explained in a publication entitled "Formation Evaluation by Analysis of Hydrocarbon Ratios" by B. 0. Pixler and puolished in Journal of Petroleum Technology, June 1969 pp. 665-670, it is possible by comparing the ratios of methane to each of several other hydrocarbon gases, such as ethane, propane, butane and pentane, to estimate whether a well will be productive and, if so, whether the well will produce oil, gas or water.
The gas chromatography technique involves taking samples of gas from the drilling mud and passing that gas through special columns filled with materials that allow different gases to flow at different rates. The different gases are identified according to the length of time required for them to pass through the column. Because identification of the different gas species requires a comparison of passage time through the chromatograph columns, the measurements of the gas species are not made continuously but instead they must be taken at discrete intervals from two to four minutes apart. As a result it has not previously been possible to ascertain hydrocarbon species ratios on a continuous basis during a drilling operation; and therefore a complete analysis of the formations along the well has not been obtainable during a drilling operation. Also, the prior art technique did not permit one to ascertain changes in the relative concentrations of different hydrocarbon species at the earliest possible time during the drilling operation. As a result, the drilling operator could not be certain that he had not drilled through a productive stratum or that he was approaching a high pressure fluid region that might produce a dangerous "kick" or even a blowout of the well.
A further disadvantage of the prior art chromatographic gas analysis technique results from the fact that it is not possible to separate all of the hydrocarbon gas from the returning mud and therefore it is not possible with chromatographic analysis to ascertain the actual concentration of any species in the mud.
The prior art also fails to provide in an integrated, self-contained system, the adaptability to switch quickly between various types of tests and the various purges, zero settings and calibration of test elements and backwash of gas separation devices that are required for continuous gas analysis during the drilling of oil and gas producing wells.